I hate the word...more of I hate what is behind it, for myself (more on that here), for those I love and those that I counsel. It's not something that is easily explained if it's something you have never struggled with.
It's even harder to explain to someone who thinks they understand it. I was told once by a mother whose son struggled with depression that she experienced it, too, at times. She explained how when she was "depressed" would cry and talk to her spouse, get over it, and then not struggle with it again for a few months. She was totally serious. And I was shocked. And hurt for the son, as to the little understanding he would really get from his mother. Not that she meant anything negative from it, she just didn't have a clue.
How do you explain the pain of the feeling of pointlessness, meaninglessness, even though you may know that in reality it is not true, but every cell within you screams otherwise. How you don't really want to feel that you are falling into a dark pit, it's not something you are just choosing to do. To sob uncontrollably at things that you know are not "cry worthy". To not understand why some days are okay and other days are such a struggle to not climb back into bed.